Cell reprogramming leaves a "footprint" behind

Reprogramming adult cells to
recapture their youthful "can-do-it-all" attitude
appears to leave an indelible mark, a team of
Salk researchers including Joseph Ecker and
Ronald Evans has found. When the scientists
scoured the epigenomes of so-called induced
pluripotent stem (iPS) cells base by base, they
discovered a consistent pattern of reprogramming
errors. What's more, these incompletely
or inadequately reprogrammed hotspots are
maintained when iPS cells are differentiated
into a more specialized cell type, providing
what the researchers dubbed an iPS cell-
specific signature.

"We can tell by looking at these hotspots
whether a cell is an iPS cell or an embryonic
stem cell," says Ecker. "But we don't know yet
what it means for their self-renewal or differentiation
potential."

These findings, published in Nature, confirm
that iPS cells, which by all appearances look
and act like embryonic stem (ES) cells, differ
in certain aspects from their embryonic cousins,
emphasizing that further research will be
necessary before they can rightfully take
embryonic stem cells' place.

The fact that reprogramming of somatic
(body) cells does not pose the same ethical
quandaries as working with stem cells isolated
from eggs or embryos prompted Japanese and
U.S. scientists to develop human iPS cells that
are just as potent as human embryonic stem
cells, with the hope that one day, iPS cell technology
can be applied to regenerative medicine.
But Ecker's study makes it clear that before
cells derived from iPS cells can be used to
repair tissue damaged through disease or injury,
some remaining questions have to be solved.
"Embryonic stem cells are considered the gold
standard for pluripotency," says Ecker. "So we
need to know whether—and if so, how—iPS
cells differ from ES cells."